Abstract: This project addresses our Center for Populafion Health and Health Disparities (CPHHD) application's overarching theme of understanding and preventing breast cancer precursors, morbidity, and mortality among Latinas by aiming to better understand the underlying biology of breast cancer in these women. Disparities in breast cancer survival befi^'een racial and ethnic groups have some well-documented social and economic causes. It is possible that differences in tumor biology between groups also contribute to differences in breast cancer outcomes. Recent gene expression profiling of breast cancer has deepened our understanding of this very heterogeneous disease and led to a new paradigm for the development of subtypespecific therapies and prevenfion strategies. However, the gene expression subtyping of breast cancer is based entirely on data generated by studies of non-Hispanic white women. The disparity created by the lack of biological data In minority women will continue to impede the implementafion of new approaches to prevention and treatment in these diverse populafions. As new treatments directed at specific subtypes become available it is essential that expression data from Hispanic women is also availableboth to identify associations specific to Hispanic women and to assess the relevance of data generated in studies of non-Hispanic women. To better understand the biological subtypes of breast cancer in Hispanic women and the relationship of genetics and environmental influences (risk factors) to the development of specific breast tumor subtypes, we will conduct a study of 615 Hispanic women based primarily on the breast cancer cases outlined in Project 3. Our primary aim is to: 1. Determine the relationship between tumor subtype and ancestry in Hispanic women with breast cancer. This will be accomplished by: a) determining the tumor subtype by gene expression array b) determining ancestry through ancestry informafive markers (AIMs) c) comparing tumor subtype with individual ancestry as defined through the AIMs. Our secondary aims are to 2. Explore the relationship between established risk factors and breast tumor subtypes in Hispanic women. 3. Explore expression of genes involved in tumor-associated pathways in the tumors of Hispanic women and the relationship of these pathways with ancestry and risk factors.